Geochemists have found evidence of animals living on Earth at least 635 million years ago – nudging back the oldest fossil records by tens of millions of years. Their research is published in this week’s Nature [subscription required].

“It is, definitively, the earliest evidence for animals,” geochemist Gordon Love, of the University of California, Riverside, tells Wired.

The animals in question were demosponges, and were tracked by a characteristic chemical marker, a 30-carbon steroid called 24-isopropylcholestane (24-ipc), which they left behind in sedimentary rocks in a salt basin in south Oman.

These compounds are “40 to 50 million years earlier than any fossil sterols seen before”, Love told Chemistry World.

They show the sponges must have lived before the Marinoan ice age, the second great freeze of the Cryogenian period (and also sometimes referred to as the time of ‘Snowball Earth’). That’s some 100 million years before an evolutionary growth spurt recorded by a boom in the fossil record, the so-called Cambrian Explosion.

Kevin Peterson, of Dartmouth College, had hypothesized such ancient sponges using molecular clock methods, based on tracing back the genes of their modern descendants, National Geographic notes. “To see a robust, geochemical record of a tremendous amount of sponge mass at this time is very exciting,” Peterson told them.

As Jochen Brocks, of the Australian National University in Canberra, tells ScienceNews, there’s evidence that eukaryotes existed some 1.9 billion years ago, but multicellular animals didn’t appear until much later. And sponges may have oxygenated the deep oceans to pave the way for more life. “Sponges might have filtered all the crap away, might have been the cleaner of the oceans,” Brocks says.

Image: Marinoan age (around 635Myr old) rock unit of calcium carbonate deposited on rocks of glacial origin.The oldest sponge steroids detected in sedimentary rocks underlie this cap carbonate, so pre-date the end of the Marinoan glaciations/David Fik